How to Find the Right Replacement Seal for Your Pump Model

Information 1: Pump Brand and Model

The pump nameplate is the fastest starting point, and it is usually bolted to the casing or the motor. It carries the brand, the model and often the impeller size and pump serial number. For many common pump ranges — Grundfos, KSB, Wilo, Ebara, Sulzer, Flowserve and similar — a supplier with good cross-reference data can look up the standard seal size directly from the model. Brand alone is not enough: a CR 10 is not a CR 32, and the seal size changes with the pump frame.

Photograph the nameplate. If it is painted over or corroded, clean it gently with a scraper and a wire brush and take the photo in good light. The serial number matters more than most people think: pump manufacturers changed seal chambers on the same model over the years, and the serial number lets a supplier check which generation your pump is. If the nameplate is missing, move to the next source of information — the old seal itself.

Also check the pump manual or datasheet if you have them; the seal section usually states the seal chamber dimensions. And note whether the pump has a shaft sleeve: pumps with sleeves take seals sized to the sleeve diameter, which may differ from the bare shaft. Write everything down — model, serial number, impeller size — before you take the pump apart.

Information 2: The Old Seal Part Number

If you have the old seal, read the part number before you discard it. Numbers are usually stamped or laser-marked on the gland, the sleeve, the drive collar or the outer diameter of the stationary face. Even a seal that has run for years usually keeps a legible number somewhere; rotate the seal and look at every flat surface. Clean the markings with solvent and a cloth — grime hides numbers well.

Write the number down exactly as printed, including dashes, letters and suffixes. The suffix often carries meaning: a different elastomer, a different face material or a different spring. If the number is damaged or unreadable, record what you can — the type prefix alone can identify the seal family, and the size can always be measured later, so a partial number is still a useful start.

And remember that the part number identifies the seal, not the pump; the number on the seal is worth more than the number on the pump. If the two disagree — the pump says one size and the seal says another — trust the seal and measure to be sure, because pumps get rebuilt with different shafts over their working lives. The measurement settles every argument.

One useful habit: store the old seal in a clean plastic bag with the pump details written on it until the replacement is confirmed. The seal is the one physical record of everything that was installed before — size, design, materials, even the failure mode. Once the new seal is in hand and confirmed to fit, the old one can go; until then it is worth more than any drawing.

Reading a Seal Part Number

Most mechanical seals for standard pumps are identified by two numbers: the shaft size and the bore size, often written like 35/60 or G60-35. This is the DIN 24960 / ISO 3069 convention, and it means: a seal for a 35 mm shaft in a 60 mm seal chamber bore. Once you know those two numbers, you can identify the correct size of almost any standard seal, regardless of brand.

The part number on the seal usually encodes the size, but the coding differs between manufacturers. A typical commercial code reads as: a type prefix (the seal design — single spring, unbalanced, cartridge and so on), then the shaft size, then the bore size, then material codes for the faces and elastomer. A B for faces might mean carbon against silicon carbide in one maker's code and tungsten carbide in another's — never decode a foreign part number from memory.

The practical trick: separate the size from the design. If you can read the shaft and bore numbers, you can match the size of any equivalent seal. If you can read the full code, you can match the design too. If you cannot read it at all, you can still measure the size yourself — which is what the next sections cover. Never guess: a seal of the right size but the wrong design will leak, and a seal of the wrong size will not fit at all.

Information 3: Shaft Diameter

The shaft diameter where the seal sits is the single most important dimension — it fixes the seal size. Measure it in millimeters, at the exact location the seal occupied, not at the coupling end. If the shaft has a wear groove where the old seal sat, measure on an unworn part of the shaft and tell the supplier if the shaft is grooved; you may need a sleeve or a seal with a different seat position.

Use calipers, not a tape measure — a millimeter matters. Take the reading twice, at two positions around the shaft, and use the average. Shafts for standard pumps typically run from 10 mm up to 100 mm or more, with common industrial sizes clustering between 20 and 50 mm. If the pump is old, check whether it has a sleeve before you measure, because the sleeve diameter is the number the seal cares about.

A shaft that measures 34.9 mm is a 35 mm shaft, but a shaft that measures 34.2 mm has worn and needs attention before it will seal again. Record the measurement and the location where you took it — the seal seat, not the coupling — so the supplier reads the same picture you do, and so the next person who replaces this seal does not re-measure from scratch.

Also record whether the shaft surface is smooth or shows wear at the seal location. If the shaft is grooved or pitted where the seal sat, the new elastomer will not seal against it reliably, and no amount of care in ordering will fix that. Mention it to the supplier — they can advise a sleeve or a seal design that sits on a fresh part of the shaft.

Information 4: Seal Bore Size

The bore size is the diameter of the opening in the pump casing where the seal sits. Together with the shaft diameter it determines which seal fits: standard seal sizes are defined by the shaft/bore pair, which is why both numbers travel together on every seal datasheet. Measure the bore with the inside jaws of a caliper, across the opening, taking two readings at right angles and averaging them.

Watch out for two traps. First, some pumps have a removable gland or seal chamber — measure the actual chamber, not the cover plate. Second, if the bore is chamfered or stepped, measure the main cylindrical section, and note any counterbore depth if the seal drawing needs it. A bore that reads 59.8 mm with the caliper is a 60 mm bore; measure three times if the reading looks odd before you trust it.

Measuring with Calipers

A digital caliper is the right tool for both shaft and bore measurements, and a decent one costs little. Set the caliper to zero with the jaws closed, clean the measuring surfaces, and make sure the shaft is clean at the measuring point. For the shaft, use the outside jaws; for the bore, use the inside jaws at the top of the caliper. Take each reading twice and average — worn or dirty surfaces give readings that drift.

Common errors: measuring a worn or corroded section of shaft and getting a diameter that is too small; measuring the bore at a chamfered edge; using a tape measure, which cannot resolve half a millimeter; and measuring the gland instead of the bore — the gland outside diameter is not the bore size. Measure where the seal actually seals, nothing else.

If you do not have calipers, a vernier caliper from a hardware store works, or ask a machinist friend — but do not substitute a ruler and a guess. If your numbers look unusual, a 44.5 mm shaft, say, the measurement is suspect: standard seal sizes step in whole millimeters for shafts and in 5 mm steps for bores in most industrial ranges.

Photos That Help

A good photo set lets a supplier confirm the fit without a single dimension being wrong. Photograph the pump nameplate, the old seal from both sides so the faces are visible, the stamped part number, the gland, the bore opening and the shaft at the seal location. Put a ruler or a coin next to parts in close-up shots — a photo without scale is decoration.

• Pump nameplate, clean and readable.
• Old seal, both sides, faces visible.
• Part number or type markings, close-up.
• Seal chamber bore with a scale next to it.
• Shaft at the seal location with a scale.
• Any damage: chipped faces, corroded shaft, broken springs.

Take the photos in daylight or with flash, from straight on rather than at an angle, and keep them sharp. If you send photos plus the measured dimensions, a supplier can usually confirm the seal size and design on the first exchange — which shortens the whole process by days compared to an email that says only that you need a seal for your pump.

What to Send a Supplier

When you contact a supplier, send everything in one message: pump brand and model, shaft diameter, bore size, the old seal part number if you have it, media, temperature, pressure and quantity. Add the photos from the previous section. A complete first message gets a complete quote — an incomplete one gets a round of questions and a slower answer, and every round-trip costs you days.

• Pump brand, model and serial number.
• Shaft diameter and bore size in millimeters.
• Old seal part number and all markings.
• Media, temperature and pressure.
• Flush or piping plan, if fitted.
• Quantity and target delivery date.
• Photos with a scale.

A serious supplier will confirm the fit before quoting — they will ask for the dimensions if you did not send them, and they may ask about the media even when the dimensions are complete. That is the sign of a supplier who wants the seal to work. If a supplier quotes instantly without asking for dimensions, treat it as a warning sign, which leads us to the next section.

Red Flags When Choosing a Supplier

The biggest red flag in this industry is a quote that never asked for the shaft and bore size. A seal is sized by those two numbers; a supplier who quotes without them is quoting blind, and a universal seal that fits anything does not exist. Equally suspicious: no questions about the media, no request for photos, and a price far below every other quote — cheap seals that fail in weeks are not cheap.

• Quotes without asking shaft/bore size or media.
• Fits-everything or one-size-for-all-pumps claims.
• No fit confirmation, no drawings, no material spec.
• Vague lead times that depend on nothing in particular.
• Prices far below market with no explanation.
• No answer when you ask about QC or testing.

None of this means you should demand a fortress of paperwork from a small supplier of standard seals — a water-pump seal does not need a test certificate. But the basics are not optional: the supplier must know the size, the media and the operating conditions before quoting, and should be willing to confirm the fit in writing. A supplier who does that is thinking about whether the seal will work, which is exactly what you are paying for.

FAQ

Q: Can I identify the seal from the pump model alone?
A: Usually yes for common pump ranges — the model plus serial number lets a supplier look up the standard seal size. For older or modified pumps, verify with a measurement.

Q: What if the shaft measures between sizes?
A: Standard shafts are whole millimeters. A reading like 34.9 mm is a 35 mm shaft; a reading like 34.2 mm means wear — fix the shaft or fit a sleeve before installing a new seal.

Q: What if the old seal has no markings at all?
A: Measure the shaft and bore and send photos with a scale. Size plus design details from the photos is usually enough to match an equivalent.

Q: Do I need to remove the pump to measure the bore?
A: Not usually — with the gland off, the bore opening is accessible and measurable in place. Remove the seal first so nothing blocks the caliper jaws.

Q: Can you match a seal I bought from another manufacturer?
A: Yes — with the shaft and bore sizes plus photos of the old seal, we match an equivalent from our range. Send the part number if you have it; we cross-reference it.

Q: What if my pump has a shaft sleeve?
A: Measure the sleeve diameter where the seal rides, not the bare shaft. The seal seals against the sleeve, so the sleeve size is the size that matters.

Q: How accurate do my measurements need to be?
A: Within about 0.1 mm. Standard sizes step in whole millimeters for shafts, so a reading of 34.9 mm is safely a 35 mm shaft — but 34.2 mm is a worn shaft, not a size. When in doubt, measure twice.